In radiation therapy (radiotherapy) a part of the tissue of a patient is irradiated with ionizing radiation in order to change the tissue part or a surrounding area including the tissue part. An external radiotherapy is known that includes an irradiation of the body of the patient from outside the body. Also known is an internal radiotherapy by radiation sources that include radioactive substances. The radiation sources are introduced into the body of the patient in order to damage or to destroy the tissue part locally in the body of the patient.
A radiotherapy may be planned and/or monitored by medical imaging methods. A radiotherapy plan based on a medical imaging dataset of the patient is created. Computed tomography image datasets (CT image datasets) are used. On the basis of the CT image datasets, the target volume of the radiotherapy may be defined and a surrounding tissue sensitive to radiation may be localized. The intensity values of the image voxels of the image data (measured in Hounsfield Units) are a good approximation for mapping an electron density at the corresponding location in the body of the patient, since the intensity value of an image voxel is based on an absorption of the x-ray radiation at the associated location. The CT image datasets may be converted for the radiotherapy planning into an electron density map. Since in radiotherapy the cross-section of the interaction of the radiation correlates in a positive way with the electron density in the body, the attenuation of the radiation during its passage through the body may be computed from the CT image datasets.
Other imaging methods with a better soft tissue contrast may be used in radiotherapy planning in order to make possible an improved identification of target organs and/or organs at risk. One such method is the recording of magnetic resonance image datasets (abbreviated to MR image dataset) using a magnetic resonance device. Since image contrasts of the MR image dataset do not have any physical relationship to electron density, no direct conclusions may be drawn therefrom about the electron density and thus the photon attenuation in the patient. For example, both bone and also air regions do not show any signal in MR contrasts and are displayed dark in an MR image dataset, although the regions have a different electron density and thus a different photon attenuation. For planning of a radiotherapy, in addition to the MR image dataset, a CT dataset is determined, that provides electron densities with the necessary precision for the radiotherapy. However, different positions of the patient and changes in the anatomy of the patient may be taken into account between the two examinations (e.g. changes in volume from air breathed in). The corresponding resources, e.g. CT devices and trained operating personnel, may be retained and available.
MR image datasets may be used for radiotherapy planning and for dispensing with the recording of additional CT image datasets (exclusively magnetic resonance-based radiotherapy planning, “MR Only Radiotherapy Planning”, abbreviated to MRORTP), and may be used to define synthetic electron density maps without the use of CT image datasets of the examined patient. A synthetic electron density map needed for radiotherapy planning and dose calculation is only been able to be determined from the MR image dataset with high algorithmic outlay and with susceptibility to errors.
Publication WO 2015/171056 A1 discloses that a synthetic electron density map may be determined on the basis of an MR image dataset. The MR image dataset is initially segmented and an electron density map already known beforehand is converted into a synthetic electron density map by applying a transformation (e.g. a registration) based on the segmented MR image dataset.